The Collaborative International Dictionary of English v.0.48:

Sedate \Se*date"\, a. [L. sedatus, p. p. of sedare, sedatum, to
   allay, calm, causative of sedere to sit. See Sit.]
   Undisturbed by passion or caprice; calm; tranquil; serene;
   not passionate or giddy; composed; staid; as, a sedate soul,
   mind, or temper.
   [1913 Webster]

         Disputation carries away the mind from that calm and
         sedate temper which is so necessary to contemplate
         truth.                                   --I. Watts.
   [1913 Webster]

         Whatsoever we feel and know
         Too sedate for outward show.             --Wordsworth.
   [1913 Webster]

   Syn: Settled; composed; calm; quiet; tranquil; still; serene;
        unruffled; undisturbed; contemplative; sober; serious.
        [1913 Webster] -- Se*date"ly, adv. -- Se*date"ness,
        n.
        [1913 Webster]

WordNet (r) 3.0 (2006):

sedateness
    n 1: a trait of dignified seriousness [syn: sedateness,
         staidness, solemnity, solemness]